1. Field of the Invention
This invention relates to pseudo noise (PN) spread sprectum communication systems, and more particularly, to apparatus and a method for generating a multi-user code for fast acquisition that appears to unfriendly parties to be a more complex acquisition code.
2. Description of the Prior Art
Multiple-access communication systems refer to systems which permit multiple users to receive communications from a single transmitter. A typical example of such a system involves a reconnaissance vehicle which has collected substantial information to be unloaded or communicated to a plurality of ground stations over a short period of time, but in the relatively secure manner. Under such circumstances, it would be desirable for the ground stations to be capable of independently receiving a composite code which contains a quadrature or acquisition code with no data, and an in-phase or track code which does contain the desired data, and to acquire both codes in the fastest possible manner which also provides a predetermined degree of security.
In our U.S. Pat. No. 4,809,295, we described a "code lengthening system" which describes an apparatus and method for generating and acquiring in a minimum of time a lengthened PN composite code. The receiving apparatus in this patent was designed to transmit signals back to the transmitter during the acquisition of the PN composite code. This mode of operation does not lend itself to practical use in a multiple access environment.
In our U.S. Pat. No. 4,225,935, apparatus is described for generating and receiving linear composite codes which include a non-linear composite code offering greater security than a linear composite code alone and may be also rapidly acquired. While the invention disclosed in this U.S. Pat. No. 4,225,935 is not embodied in the present invention, it could be included to enhance the security of the present invention system. The present invention does employ the techniques of combining a plurality of linear PN component codes which are relatively prime with respect to each other and have an approximately equal number of binary ONES and ZEROES. With respect to the linear MAJ and MOD composite codes created from the components, these codes also possess similar correlation properties.
It would be desirable to provide a multiple access code acquisition system which incorporates the prior art composite codes but is not burdened by the disadvantage of having to transmit information back to the airborne transmitter during acquisition of the composite code.